The present invention refers to a starting system for an induction electric motor, particularly of the type used in refrigeration hermetic compressors.
In the conventional constructions, an electric motor of the type mentioned above has a main coil and an auxiliary coil, which are supplied with electric current by an electric current source through a starting circuit of the electric motor usually comprising a starting relay or a thermistor (PTC) mounted in a case to be electrically connected to the external wall of the compressor shell in which this electric motor operates.
In these constructions, a terminal of each of the main and auxiliary coils of this electric motor is directly connected to the electric current source through a switch element of this starting circuit, whereas another terminal of the auxiliary coil is electrically connected to said electric current source through the starting relay or PTC.
In the construction using a starting relay, the excitation of a winding thereof leads to the magnetic closing of its contacts, allowing current to pass to the auxiliary coil of the electric motor. This energization condition remains while the current through the winding of the relay is sufficient to maintain the closing condition of its contacts. In general, just after start, when the current drops in the main coil of the motor, the current through the winding of the relay is substantially reduced, causing, not always in the desired occasion, the opening of the relay contacts and the interruption of the current passing through the auxiliary coil of the motor.
The use of a starting relay further has some disadvantages, such as the possibility of occurring melting of the relay contacts in a situation of high current discharge, possibility of electric interferences and sparking.
In the construction employing a starting PTC, upon start of the electric motor, the PTC allows electric current to pass to the auxiliary coil, this condition being temporarily maintained in function of the PTC characteristics. With the passage of electric current, the PTC temperature is altered and increases progressively. Upon heating, the PTC resistance is gradually increased, progressively impairing the passage of the electric current and consequently reducing the passage of said current to the auxiliary coil, such reduction occurring until the passage of the current to the auxiliary coil has been blocked. However, in practice, although gradually reducing the current through the auxiliary coil of the motor, the PTC resistance does not effectively interrupt the current supply to the auxiliary coil, allowing a residual current to remain therethrough, resulting in energetic consumption of the electric motor. Moreover, the existence of current flowing through the PTC keeps the latter heated, impairing new starts of the electric motor, since a restart should only occur when the PTC has a determined temperature in which there is no significant resistance to the passage of the electric current.
Besides the inconveniences resulting from the provision of a starting relay or a PTC for achieving the start of the electric motor, this conventional construction of providing the starting circuit with such elements externally to the compressor causes some inconveniences, such as undue heating of the case and consequently of the components mounted therewithin, difficulty and discomfort for assembling said case to the compressor and exposure of said case to impacts when the compressor is moved, for example during shipping, which may cause an undue disconnection of the case containing the starting circuit from the compressor shell.
Thus, it is an objective of the present invention to provide a starting system for an electric motor, which allows to dispense the use of a starting relay or a starting PTC in the starting circuit of said motor, in order to avoid the inconveniences resulting from the use of these components.
Another objective of the present invention is to provide a starting system for an electric motor of the above mentioned type, which may be mounted to a hermetic compressor without the inconveniences known in the prior art.
These and other objectives of the present invention are achieved by a starting system for an electric motor of the type comprising a main coil and an auxiliary coil, which are supplied by an electric current source, said system comprising: a switch provided between the electric current source and the auxiliary coil and having an open condition, in which the auxiliary coil remains deenergized, and a closed condition for energization of the auxiliary coil; and an actuating means provided inside the electric motor and submitted to a magnetic field produced by the main coil upon its energization for operation of the electric motor, said actuating means closing the switch when the current passing through the main coil produces a magnetic field with an intensity superior to a determined minimum value, and opening the switch when the current through the main coil produces a magnetic field presenting an intensity with the determined minimum value.